What Makes Eyes Different Colors?

What a baby is going to look like depends on the 46 genes in his body cells. Half of these genes he inherits from his mother, and half from his father. Some of these genes determine his eye color. All eyes contain blue cells, but some people have other colored cells as well so that their eyes look green or gray or brown or hazel.

Babies all have blue eyes because the other cells, if there are any, do not grow until the baby is about two. Sometimes people have eyes which are partly blue and partly brown or green. This is because half of their eye has only the usual blue color, while the other half has some other color as well.

When two blue-eyed people have a baby only blue cell-genes are being passed on, so all their babies will have blue eyes. But when one parent has brown eyes, a mixture of blue and brown eyes is being inherited, and the color of the babies’ eyes will depend on whether there are enough brown genes to dominate the blue ones. Even two brown-eyed parents may not have enough brown genes between them to have a brown-eyed baby.

Eye color is the result of variations in the amount of melanin, a pigment found in the front part of the iris of the eye. The lack of this pigment results in blue eyes, some pigment gives green and lots of pigment gives brown eyes. All of the different shades of eye color happen the same way. Blue-green eyes have an amount of melanin between green and blue, hazel eyes have an amount of pigment between green and brown, etc.

Some people have eyes that have different colored patches. For example, blue eyes with a green or brown circle around the pupil are pretty common. In these eyes, different parts of the iris make different amounts of melanin. What we don’t have yet is a good handle on how this all happens genetically. Scientists have a pretty good model based on two genes that can help explain blue, green, and brown eyes. This is the model we based our eye color calculator on.

Scientists have even found the key gene, OCA2, which can explain why some people have brown eyes and some people don’t. Despite some work, scientists haven’t been able to find the key gene involved in green eyes. This is most likely because there is more than one gene.

A new study has identified three new genes that affect eye color. Scientists don’t know what each of these genes exactly do and they don’t know how to use them in eye color calculators just yet. But given what we know about eye color, we can make some pretty good guesses about what these genes probably do…

Most likely these genes are either responsible for making melanin themselves or they control how much melanin other genes make. Either way you end up with different shades of eye color based on the combination of genes that you have.

Content for this question contributed by Jeff Baxter, resident of Cleveland Heights, Cuyahoga County, Ohio, USA